In 1970, the assignee of the present application developed and placed on the market a machine operable to swiftly and efficiently reduce whole trees, with limbs and branches attached, to wood chips. See U.S. Pat. No. 3,661,333. Although developed with other purposes in mind, it soon became apparent that this particular machine made feasible, for the first time, selective thinning programs for large areas of forests over-grown with scrub and unmerchantible trees which were choking out or hindering the growth of pines, hardwoods and other desirable timber. Previously, selective thinning of such forests on any reasonable scale was difficult or economically impractical because of the problem of disposing of the brush and other trees felled during the thinning process.
This problem was solved by the machine of U.S. Pat. No. 3,661,333--that machine reduced this scrub and felled trees swiftly to wood chips and in the process blew the chips into vans which could be used to transport the chips out of the immediate area. The chips so produced were used as mulch or, after suitable separating processes, found some commercial usage in paper-making or particle board making operations. However, the potential supply of wood chips from such selective thinning operations far exceeded the commercial demand for such chips.
It was then proposed that such chips would constitute an excellent source of fuel. The trees from which the chips were produced represented a self-replenishing source in that the species removed during the selective thinning operation, of which, aspen is a typical example, re-establish themselves quite rapidly. The species sacrificed in the selected thinning operation have little economic or ecological value. In view of the oil crisis, these facts suggested that wood chips might well be employed as a fuel for electric power generating plants.
Investigations have led to the conclusion that there are many areas in the United States where sufficient undesirable and unmerchantible trees fluorish to the point where selective thinning programs would produce a sufficient quantity of wood chips to provide a permanent source of supply of fuel to operate an electrical generating plant adequate to meet the needs of a small town and that this source would regenerate itself at a rate exceeding the rate of consumption.
One problem addressed by the present invention is the fact that substantially all of the wood chips produced in a selective thinning operation of the type referred to above are produced from green wood--that is from living trees whose wood contains a substantial amount of moisture. Green chips do not burn as readily as dry chips. Because the green chips typically are produced in very large quantities, outdoor storage in large piles is the only practical method, and such storage is not conducive to the drying of the chips, particularly those in the interior of the pile. Another problem addressed is that of the release of contaminants in the flue gas when the chips are burned alone or as part of a fuel mix.
In accordance with the present invention, hot combustion gases created by the burning of chips are employed to heat, and thus dry, subsequent chips as they are being fed to the furnace and the chips are simultaneously used to filter out contaminants.